Battery pack dehumidifier with active reactivation system

ABSTRACT

A battery pack dehumidifier system for controlling the relative humidity within a battery pack enclosure is provided, the dehumidifier system including a desiccant that absorbs/adsorbs water vapor from the battery pack enclosure. The system heats and reactivates the desiccant at predetermined time intervals or when the humidity within the system reaches a preset level, thereby allowing the desiccant to regain its potential for absorbing/adsorbing water vapor.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims benefit of the filing date of U.S. ProvisionalPatent Application Ser. No. 61/495,713, filed 10 Jun. 2011, thedisclosure of which is incorporated herein by reference for any and allpurposes.

FIELD OF THE INVENTION

The present invention relates generally to battery cells and, moreparticularly, to a system for extending the life of the cells within abattery pack by controlling the humidity within the pack.

BACKGROUND OF THE INVENTION

Batteries can be broadly classified into primary and secondarybatteries. Primary batteries, also referred to as disposable batteries,are intended to be used until depleted, after which they are simplyreplaced with one or more new batteries. Secondary batteries, morecommonly referred to as rechargeable batteries, are capable of beingrepeatedly recharged and reused, therefore offering economic,environmental and ease-of-use benefits compared to a disposable battery.

Although rechargeable batteries provide a much longer service life thandisposable batteries, their service life is not unlimited. There are anumber of factors that limit battery service life, including; (i) thenumber of recharging cycles the battery has been subjected to, (ii) therate of charging (i.e., slow trickle charge versus fast charge), (iii)the level of charging (i.e., 75% of full charge, full charge,over-charged, etc.), (iv) the level of discharge prior to charging(i.e., completely depleted, still charged to a low level, etc.), (v) thestorage temperature of the battery during non-use, and (vi) thetemperature of the battery during use. Additionally, battery internalmechanical and chemical instability can adversely affect battery servicelife.

In general, the battery chemistries used in secondary cells are lessstable than those used in primary cells. As a result, secondary cellsoften require special handling during fabrication. For example,lithium-ion batteries are typically manufactured in humidity-controlled,dry rooms and sealed to minimize subsequent water contamination.Batteries may also be manufactured in an inert atmosphere, therebypreventing cell contamination from any of a variety of reactant gases.

Batteries are sealed to prevent leakage and/or contamination from water,oxygen, carbon dioxide, or other materials. Unfortunately, battery sealsare imperfect, thereby allowing gradual contamination and degradation ofthe batteries. One approach to overcoming this problem is to improve thebattery seals. For example, U.S. Patent Application Publication No.2003/0096162 discloses a hermetic seal that is compatible with corrosiveelectrolytes such as the lithium-ion electrolyte used in a lithium cell.Although improved battery seals offer one approach to overcomingcontamination issues, this approach typically requires differentsolutions depending upon the cell chemistry and geometry in question.

Another approach to preventing cell contamination from water is tocontrol the relative humidity within the battery pack itself, forexample using passive desiccant bags. This approach is described inco-pending U.S. patent application Ser. No. 12/386,684. Unfortunately,as sources of moisture production within the battery pack persistthroughout the life of the pack, these types of passive desiccant bagseventually reach a saturation point at which point they can no longerabsorb additional moisture. As a result, once saturation is reached, thedesiccant bags must be replaced or the relative humidity within the packwill rise to a level that may result in dewing events. Desiccant bagreplacement introduces undesirable service intervals throughout the lifeof the battery pack in order to replace the bags as they becomesaturated.

Accordingly, what is needed is a maintenance-free solution to batterypack humidity control which is operable throughout the service life ofthe battery pack. The present invention provides such a system.

SUMMARY OF THE INVENTION

A battery pack dehumidifier system for controlling the relative humiditywithin a battery pack enclosure is provided, the dehumidifier systemincluding a desiccant enclosure, a desiccant contained within thedesiccant enclosure, a first air passageway that couples the desiccantenclosure to the battery pack enclosure and allows water vapor to passfrom the battery pack enclosure to the desiccant enclosure where it isabsorbed and adsorbed by the desiccant, a controller configured toestimate when the desiccant loses absorption/adsorption efficacy atwhich point the controller outputs a first control signal, a heaterassembly configured to heat and reactivate the desiccant upon receipt ofthe first control signal, and a second air passageway coupling thedesiccant enclosure to a volume of air outside of the battery packenclosure (e.g., the ambient environment) that allows moisture releasedduring desiccant heating and reactivation to pass out of the system.

The desiccant enclosure may be mounted within the battery packenclosure; alternately, the desiccant enclosure may be mounted to theoutside of the battery pack enclosure; alternately, the desiccantenclosure may be mounted remotely from the battery pack enclosure. Thedehumidifier system may include a fan to circulate air between thebattery pack enclosure and the desiccant enclosure. The dehumidifiersystem may include valves that close the first air passageway and openthe second air passageway during desiccant heating and reactivation, andopen the first air passageway and close the second air passageway duringpre- and post-heating and reactivation.

The controller may be configured to output a second control signal aftera preset period of time has passed since outputting the first controlsignal, whereby the heater assembly terminates heating and reactivationupon receipt of the second control signal. The system may include atleast one humidity sensor, for example mounted within the battery packenclosure or the desiccant enclosure, to monitor humidity levels,whereby the controller outputs the first control signal when themonitored humidity exceeds a preset level. The controller may use atimer to determine when to output the first control signal, specificallyoutputting the first control signal after a preset period of time haspassed since system initialization or since the controller last outputthe first or second control signal.

The heater assembly may include a heating element positioned within thedesiccant; alternately, the heater assembly may include a heatingelement that is positioned within a heating chamber that is separatefrom the desiccant enclosure, whereby air heated within the heatingchamber is driven through the desiccant enclosure during heating andreactivation of the desiccant; alternately, the heater assembly mayinclude a heating element positioned within a heating chamber locatedwithin the desiccant enclosure but separate from the desiccant. Theheating assembly may include a fan to drive heated air through thedesiccant and desiccant enclosure. The heating assembly may include aplurality of thermally conductive structures (e.g., fins) that arepositioned throughout the desiccant enclosure and within the desiccant,whereby the thermally conductive structures are heated by the heatingelement and transfer heat from the heating element to the desiccant.

A further understanding of the nature and advantages of the presentinvention may be realized by reference to the remaining portions of thespecification and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a preferred embodiment of a battery packenclosure;

FIG. 2 provides a schematic illustration of a passive battery packdehumidifier in which the desiccant is mounted within the packenclosure;

FIG. 3 provides a schematic illustration of a passive battery packdehumidifier in which the desiccant is mounted outside of the packenclosure;

FIG. 4 provides a schematic illustration of an active battery packdehumidifier based on the configuration shown in FIG. 2;

FIG. 5 provides a schematic illustration of an active battery packdehumidifier based on the configuration shown in FIG. 3;

FIG. 6 provides a system level diagram of a dehumidifier system mountedwithin an electric vehicle;

FIG. 7 illustrates an exemplary dehumidifier system in which the heateris mounted in close proximity to the desiccant;

FIG. 8 illustrates an exemplary dehumidifier system in which the heateris mounted within a heating chamber separate from the desiccant chamber;

FIG. 9 illustrates an exemplary dehumidifier system in which the heateris mounted within the desiccant;

FIG. 10 illustrates a modification of the dehumidifier system shown inFIG. 9 in which multiple heating elements are mounted within thedesiccant;

FIG. 11 illustrates an exemplary dehumidifier system in which thermallyconductive fins are used to achieve more uniform heating of thedesiccant;

FIG. 12 illustrates an exemplary fin for use in a system such as thatshown in FIG. 11;

FIG. 13 illustrates an exemplary fin that includes a plurality ofapertures for use in a system such as that shown in FIG. 11;

FIG. 14 illustrates a crisscross pattern of thermal conductors for usein a system such as that shown in FIG. 11;

FIG. 15 illustrates a process for determining when to activate thedesiccant reactivation system of the invention based on timing;

FIG. 16 illustrates a modified process based on the procedure of FIG.15;

FIG. 17 illustrates a modified process based on the procedure of FIG. 15in which desiccant reactivation is performed during a vehicleout-of-service period;

FIG. 18 illustrates a process for determining when to activate thedesiccant reactivation system of the invention based on monitoredhumidity;

FIG. 19 illustrates a modified process based on the procedure of FIG.18; and

FIG. 20 illustrates a modified process based on the procedure of FIG. 18in which desiccant reactivation is performed during a vehicleout-of-service period.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

In the following text, the terms “battery”, “cell”, and “battery cell”may be used interchangeably and may refer to any of a variety ofdifferent cell types, chemistries and configurations including, but notlimited to, lithium ion (e.g., lithium iron phosphate, lithium cobaltoxide, other lithium metal oxides, etc.), lithium ion polymer, nickelmetal hydride, nickel cadmium, nickel hydrogen, nickel zinc, silverzinc, or other battery type/configuration. The term “battery pack” asused herein refers to multiple individual batteries contained within asingle piece or multi-piece housing, the individual batterieselectrically interconnected to achieve the desired voltage and capacityfor a particular application. The term “electric vehicle” as used hereinmay refer to an all-electric vehicle, also referred to as an EV, aplug-in hybrid vehicle, also referred to as a PHEV, or a hybrid vehicle,also referred to as a HEV, where a hybrid vehicle refers to a vehicleutilizing multiple propulsion sources one of which is an electric drivesystem.

FIG. 1 is a perspective view of an exemplary battery pack 100 that maybe used with the invention. As shown, a plurality of individualbatteries 101 is mounted within a multi-piece enclosure that iscomprised of a lower housing member 103 and an upper housing member 105.As the invention is not limited to a particular number of cells, aspecific battery chemistry or style, or a particular interconnectconfiguration, further battery pack details in this regard are notprovided herein.

In order to minimize battery and interconnect particulate andnon-particulate (e.g., vapor) contamination, lower housing member 103and upper housing member 105 are each fabricated from a material ormaterials that are impermeable to water and water vapor, and preferablyimpermeable in general to other liquids and gases. Additionally, as thehousing members are intended to contain a plurality of cells, in someinstances hundreds or thousands of cells, the housing members arefabricated from materials capable of handling the weight of the cellsfor the intended application. For example, one or both housing members103/105 may be fabricated from a metal (e.g., aluminum, an aluminumalloy, steel, etc.) or from a plastic or a high strength, lightweightcomposite such as a carbon composite. In some instances it may benecessary to coat the material comprising the housing with animpermeable layer, e.g., a metal layer deposited on a plastic housingstructure. Such an impermeable layer may be added using any of a varietyof well-known coating techniques such as vapor deposition. The use of anadditional impermeable coating allows the selection of the material usedfor the housing members to be based on the material's mechanical andelectrical properties (e.g., high strength, low weight, high structuralrigidity, electrically non-conductive, etc.), rather than its liquid andgas impermeability.

In order to achieve the desired enclosure impermeability, a compressibleand impermeable seal 107, also referred to herein as a sealing gasket,is interposed between the complimentary and mating surfaces of lowerhousing member 103 and upper housing member 105. A portion of sealinggasket 107 is shown in FIG. 1. Those of skill in the art will recognizethat there are countless materials from which seal 107 can befabricated, exemplary materials including, but not limited to,polyurethanes, polychloroprenes, rubber-edged composite materials,coated (e.g., PVC coated) polymers, uncoated polymers, synthetic rubbers(e.g., butyl rubber), and acrylic impregnated polyurethanes.

In the exemplary battery pack 100, sealing gasket 107 is positionedbetween flange 109 of lower housing member 103 and a surface of theflat, upper housing member 105. In configurations utilizing a non-flatupper housing member, the upper housing member includes a flange that iscomplimentary to flange 109. Note that gasket 107 may be flat as shown,or utilize an alternate configuration (e.g., a circular cross-sectionprior to compression). Battery pack 100 includes means, for example aplurality of bolts 111, for compressing seal 107 and holding togetherthe housing members. Bolts 111 may also be used to attach enclosure 100to the mounting structure of the intended application, for example tothe mounting bay of an electric vehicle.

In order to protect cells 101 from environmentally induced degradation,all connections to the internal volume of enclosure 100 are preferablyhermitically sealed. Thus in exemplary battery pack 100, electricalconnections 113 are hermitically sealed to lower housing member 103 asare the coolant lines/connections 115 used to couple an active coolingsystem to the battery pack.

Although battery pack 100 is designed to prevent the intrusion of watervapor, thereby protecting cells 101, it will be appreciated that duringthe lifetime of a battery pack the batteries may still be subjected toan undesirable and potentially harmful amount of water vapor, forexample due to gasket leakage, hermitic seal leakage, coolant systemleakage, and the out-gassing of the various materials used for thecells, cell interconnects, cooling system, and internally packagedelectronics. Accordingly, battery pack 100 includes means to removewater vapor from within the pack enclosure.

Although a variety of different techniques may be used to collect andremove water vapor from the battery pack, in accordance with theinvention a desiccant is used to remove water vapor from withinenclosure 100 via absorption and/or adsorption. In battery pack 100, thedesiccant is held within a container 117 mounted within the enclosure.The battery pack may also include a pressure management system thatinsures that the pressure differential between the inner volume of theenclosure and the outside environment stays within a predeterminedrange. In pack 100, a pressure management system is included that iscomprised of one or more pressure relief valves 119, valves 119preferably being two-way valves. Pressure relief valve(s) 119 ensuresthat the pressure differential between the inner enclosure volume andthe outside environment does not become large enough to cause structuraldamage to the enclosure. Pressure differentials may be caused by thebattery pack being moved to a different altitude and thus subjected to adifferent external pressure, or may arise due to component out-gassing,battery cell venting, temperature changes, etc. In order to minimize therisk of water vapor entering into the enclosure via the relief valve,the valve has preset relief points (i.e., set points). The pressurerelief set point may be different depending upon the direction ofrelease, i.e., inward versus outward venting, or may utilize the sameset point. A typical pressure relief set point is 1 psi in eitherdirection.

It will be appreciated that there are a variety of ways that the systemmay be configured in order to expose the environment within the batterypack, and thus the cells within the battery pack, to the desiccant. Forexample, FIGS. 2 and 3 illustrate two passive configurations in whichair from the battery pack is allowed to flow through the desiccant. Insystem 200 the desiccant container 201 is mounted within the batterypack and airflow, represented by arrows 203, is allowed to flow throughone or more desiccant container apertures (e.g., holes, slits, etc.). Inthe alternate configuration shown in FIG. 3, the desiccant container 301is mounted outside of the battery pack 300, but coupled to the batterypack such that air flow 303 is still allowed to flow freely between thebattery pack and the desiccant contained within the desiccant container.It will be appreciated that desiccant container 301 may be mountedremotely from the battery pack with air flowing between the two via oneor more air passageways/conduits.

While the design of a dehumidifier for a relatively small volume of air,i.e., a small battery pack, is straightforward, controlling the humiditywithin a large battery pack is much more challenging. Assuming the useof a passive system, preferably multiple containers of desiccant arespread throughout the battery pack, if internally mounted, or coupled tomultiple regions of the battery pack, if externally mounted, thusassuring adequate exposure of the entire volume of air within the packto desiccant. Alternately, or in addition to the use of multiplecontainers of desiccant, an active humidity control system may be used.FIGS. 4 and 5 illustrate the two basic configurations for an activecontrol system based on the passive systems shown in FIGS. 2 and 3,respectively. As illustrated, one or more fans 401 are used to withdrawair from the battery pack and force it through the desiccant.Circulating the air through the battery pack and through the desiccantimproves performance of the dehumidifier system. Active air circulationalso makes it easier to utilize a remote desiccant container that iscoupled to the battery pack via conduits. Depending upon the volume ofair within the battery pack as well as the overall size of the pack,further improvements in air circulation may be achieved by utilizinginternal battery pack walls that help to force the circulating air topass around all of the cells. For example, battery pack 400 includesinternal wall 403 and battery pack 500 includes internal wall 501. Insome configurations auxiliary circulatory fans, e.g., fans 405, locatedwithin the pack are used to further enhance air circulation. It shouldbe understood that even with an active desiccant-based dehumidifiersystem, multiple desiccant containers may be required, depending uponthe volume of air within the pack, the dimensions and configuration ofthe pack, and the amount and type of desiccant per container.

As noted above, the purpose of the desiccant-based dehumidifier system,regardless of whether the system is an active or a passive system, is toremove moisture, i.e., water vapor, from the volume of air within thebattery pack. To accomplish this task one or more venting pathways areprovided between the volume of desiccant and the internal battery packair. The amount of water vapor that is removed from the air within thepack, as well as the efficiency of this process, depends on a variety offactors including the type and volume of desiccant used, the surfacearea of the exposed desiccant, the relative saturation level of thedesiccant, the temperature of both the air within the pack and thedesiccant, and lastly the relative humidity of the battery pack air.

In order to insure that the desiccant-based dehumidifier is able tocontinue to be effective throughout the lifetime of the battery pack,the desiccant must be replaced whenever it becomes too saturated toefficiently remove water vapor from the pack. While desiccantreplacement is a viable approach to maintaining an effectivedehumidifier, such an approach is undesirable for many applications,such as an electric vehicle, due to the potential difficulty of removinga large battery pack from the vehicle and replacing the pack's desiccantas well as the desire to minimize owner inconvenience. Accordingly, thepresent invention eliminates the need for desiccant replacement byproviding a means for reconditioning the desiccant.

Desiccant reconditioning, also referred to herein as desiccantreactivation, is accomplished by drying out the desiccant through theapplication of heat and providing means for eliminating the collectedwater vapor from the system, thereby allowing the desiccant to regainits capacity for water vapor absorption/adsorption. In order to preventrecontamination of the battery pack with water vapor, during desiccantheating the airflow pathways between the desiccant and the battery packare preferably closed, thus insuring that the air that becomes saturatedwith the water vapor being driven from the desiccant is expelled outsideof the battery pack.

There are a variety of techniques that may be used to heat andrecondition the desiccant while preventing re-entry of water vapor intothe battery pack. While these techniques and systems are describedgenerally below, it will be appreciated that the specific configurationused for a particular application depends on a variety of factors. Forexample, in vehicle 600 shown in FIG. 6, desiccant-based dehumidifier601 may either be integrated within battery pack 603 or be mountedexternally to the battery pack but coupled to the pack such that airflow circulates between the pack's internal volume of air and thedesiccant container. In at least one embodiment dehumidifier system 601utilizes an internal controller 605 while in at least one alternateembodiment the dehumidifier system utilizes the vehicle control system607, thus reducing manufacturing cost and overall vehicle complexity.The controller, either internal controller 605 or system controller 607,estimates when the desiccant-based dehumidifier begins to loseabsorption/adsorption efficacy due to reaching its saturation level(i.e., its water handling capacity) at which point the controlleractivates a heating system to reactivate the desiccant. Determination ofdesiccant absorption/adsorption efficacy may be based on internalhumidity levels, timing (e.g., time since the desiccant was placed intoservice or since the last reactivation process), or based on othercriteria. Preferably dehumidifier system 601 utilizes a system dedicatedheater 609 during the reactivation process, although in at least onealternate embodiment the dehumidifier system 601 utilizes the vehicle'sthermal management system 611, while in yet another alternate embodimentthe dehumidifier system 601 utilizes heat withdrawn from the drive train613 (e.g., motor heat, transmission heat, etc.). Additionally, and asdescribed in detail below, in at least one embodiment the systemincludes one or more humidity sensors, for example mounted withinbattery pack 603 (i.e., sensor 615) and/or mounted within thedehumidifier system (i.e., sensor 617), to gauge desiccant saturation.

FIGS. 7-11 illustrate exemplary heater configurations that may be usedto heat and recondition the desiccant of the battery pack dehumidifierof the invention. It will be appreciated that while the illustratedconfigurations show the basic components used by each heater, there arenumerous variations that may be used depending upon (i) the volume andconfiguration of the battery pack, (ii) the number of dehumidifiersystems coupled to the pack, (iii) the type, volume and configuration ofthe desiccant used, and (iv) the expected humidity levels within thepack. In dehumidifier system 700, enclosure 701 containing desiccant 703is coupled to the internal battery pack such that during normaloperation airflow is freely permitted between the internal air volume ofthe battery pack and desiccant 703. As noted above, since thedehumidifier system may be mounted within, or external to, the batterypack, the system may use conduits, gratings, apertures, slits, etc. toprovide means for allowing such airflow. Additionally, and also as notedabove, one or more fans may be used to enhance air circulation. In thefollowing figures, the coupling between the battery pack (e.g., pack603) and the dehumidifier system is represented by conduits 705.

In system 700, a heating element 707 is mounted in close proximity todesiccant 703. Heating element 707, shown coupled to a heater powersource (e.g., battery) 709, is positioned close enough to desiccant 703to heat the desiccant through a combination of radiation, conduction andconvection. In at least one embodiment element 707 utilizes an electricresistive heating element (e.g., nickel-chromium wire heater). Note thatin this configuration heating element 707 is held apart from desiccant703 using a thermally conductive structure 711. In addition to beingthermally conductive, preferably structure 711 utilizes a porous designthat maximizes the transfer of heat from heater 707 to the desiccant703, while preventing desiccant 703 from falling into the heater volume713.

As water vapor evaporates from desiccant 703, it is expelled to theambient environment through one or more conduits 715. Preferably thesystem also includes means, e.g., conduit(s) 717, for drawing fresh airinto heater volume 713. In at least one embodiment, the system alsoincludes means, represented by fan 719, for forcing the heated airthrough desiccant 703, thereby enhancing and expediting the removal ofmoisture from the desiccant. Note that fan 719 may be mounted within theentrance to the container, within the air input (e.g., conduit 717),and/or within the air exhaust (e.g., conduit 715) in order to pull airthrough desiccant 703.

As previously noted, it is important that the water that evaporates fromthe desiccant during the reactivation process is not permitted tore-enter the battery pack. Accordingly in at least one preferredembodiment during the reactivation process airflow between the batterypack and the desiccant container is prevented, for example by closingvalves 721. Additionally and as noted above, preferably during thereactivation process airflow is permitted, at least to a limited extent,between the ambient environment and the desiccant in order to provide ameans of exhausting the water vapor emitted by the heated desiccant. Inthe illustrated embodiment, ambient airflow is controlled using valves723.

FIG. 8 illustrates a modification of the desiccant heating system. Insystem 800, rather than locating the heating element within desiccantcontainer 701 and within a volume adjacent to desiccant 703, heatingelement 707 is mounting outside of desiccant container 701. Preferablyin this configuration heating element 707 is mounting within a separateenclosure 801 that is coupled to desiccant container 701 via conduit 717and to an ambient air intake 803. Ambient air, heated by heater 707, isforced to flow through desiccant 703 using one or more fans 805. Theheated ambient air then dries desiccant 703, primarily throughconvection, with the water vapor being exhausted to the ambientenvironment through one or more conduits 715. It will be appreciatedthat heater chamber 801 may be mounted immediately adjacent to desiccantcontainer 701 or mounted at some distance from container 701 as shown.As in system 700, preferably one or more valves, e.g., valves 721 and723, are used to prevent water from desiccant 703 being reintroducedinto the battery pack during desiccant reactivation and to provide ameans for controlling the flow of ambient air into desiccant container701.

FIG. 9 illustrates another modification of the desiccant heating system.In system 900 heating element 707 is positioned within desiccant 703,thereby primarily drying out desiccant 703 via conduction. As in systems700 and 800, outside ambient air is drawn through the system, e.g.,flowing into the desiccant container through one or more conduits 717and flowing out of the desiccant container through one or more conduits715, in order to exhaust the water vapor rejected by the desiccantoutside of the battery pack. Also as in systems 700 and 800, preferablyvalves are used to control the flow of air between desiccant container701 and either the battery pack or the ambient environment, dependingupon whether the desiccant in container 701 is being used to absorbwater vapor from the battery pack or the water vapor is being driven outof the desiccant during the desiccant reactivation process. Note thatsystem 900 preferably includes one or more fans 901 to drive ambient airthrough the desiccant during the heating and reactivation process,thereby enhancing the drying out process.

In general, desiccant 703 performs best when it is spread into arelatively thin layer, e.g., preferably with a layer thickness of lessthan 1 inch, and even more preferably with a layer thickness of lessthan 0.5 inches. While thicker layers of desiccant are still capable ofabsorbing water vapor, absorption efficiency drops off due to the longerpathway required for the water vapor to reach all regions of the thickdesiccant layer. Fortunately for a low water content application, suchas a battery pack, water absorbed/adsorbed in the outermost desiccantlayers is gradually transferred to the underlying layers, thus providingadequate absorption/adsorption even in very thick desiccant layers(e.g., 3-5 inch thick layers).

While thick layers of desiccant may be used to absorb/adsorb water vaporfrom a battery pack, the inventors have found that the reactivation ofthick desiccant layers is difficult with a simple heater configurationsuch as those shown above in FIGS. 7-9. While part of the difficultyarises due to the need to reactivate the desiccant in a timely manner,the primary issue is one of reaching a relatively uniform temperaturewithin container 701, that temperature being great enough to reactivatethe desiccant. Given the low thermal conductivity of a typicaldesiccant, heat applied to one region of the desiccant does notadequately heat other regions, especially for large layer thicknesses.If adequate heat is not applied throughout container 701, water vapor ismerely driven from one region of desiccant to another. Inadequatedesiccant heating is especially problematic if chamber 701 is open tothe ambient environment during the reactivation process, as this wouldallow even more water vapor to be absorbed by the desiccant.Accordingly, if the thickness of the desiccant layer contained withinchamber 701 is very large, preferably a heater configuration is usedthat adequately heats the entire volume of desiccant to a temperaturegreat enough to reactivate the desiccant. It will be appreciated thatfor very large desiccant containers with large volumes of desiccant,there may be small regions within the container that are allowed toremain below the desired reactivation temperature. In general, however,at least 75% of the desiccant is raised above the reactivationtemperature, more preferably at least 80% of the desiccant is raisedabove the reactivation temperature, and still more preferably at least90%-95% of the desiccant is raised above the reactivation temperature.It should be understood that the point at which an alternate heaterconfiguration is required in order to heat the entire volume ofdesiccant depends upon a number of factors, including the type ofdesiccant, desiccant bead/grain size, packing density of the desiccantwithin container 701, volume and configuration of the container, airflow through the desiccant during drying, required reactivationtemperature, ambient temperature, etc. In general, however, theinventors have found that a heater configuration that is able touniformly heat the entire volume of desiccant is required when thedesiccant layer thickness is greater than 0.25 inches; alternately,greater than 0.5 inches; alternately, greater than 0.75 inches;alternately, greater than 1.0 inches.

FIG. 10 illustrates a modification of the system shown in FIG. 9 that isconfigured to achieve greater heating uniformity throughout the volumeof desiccant 701. As shown, system 1001 includes multiple heatingelements 707 that are located throughout the volume of desiccant,thereby providing more uniform heating of the desiccant.

FIG. 11 illustrates an alternate approach to uniformly heating thedesiccant, system 1100 utilizing a single heater 707. In system 1100,however, the heat from heater 707 is transferred throughout thedesiccant via a plurality of thermally conductive structures 1101 thatare thermally coupled to heater 707. Preferably structures 1101 arefabricated from copper, although other thermally conductive materials(e.g., aluminum) may be used. In addition to having a high thermalconductivity, structures 1101 are designed to minimize their impact onair flow through the desiccant container, thus allowing the desiccant tostill efficiently remove water vapor from the battery pack. It will beappreciated that the exact configuration for structures 1101 depend upona variety of factors including the direction of airflow during normaluse, the configuration of the system used to force air through thedesiccant during normal use and/or desiccant reactivation, the volume ofdesiccant to be heated, the size of the desiccant chamber, and whetherthe desiccant chamber is located inside or outside of the battery pack.In the embodiment illustrated in FIG. 11, heat transfer structures 1101are comprised of fins. As such, preferably the flow of air between thedesiccant chamber and the battery pack is normal to the figure.Exemplary configurations for structures 1101 include fins (FIGS. 11-12);porous plates, i.e., flat elements 1301 that include a plurality ofapertures 1303 (FIG. 13); and a crisscross pattern of thermallyconductive elements 1401 (FIG. 14).

The system of the invention can use several different techniques todetermine when to reactivate the desiccant using the disclosed heatingsystem. In the technique illustrated in FIG. 15, after the system isinitialized (e.g., first put into service) (step 1501), a system timeris also initialized (step 1503). As long as the time sinceinitialization is less than a preset time period (step 1505), it isassumed that the moisture handling capacity of the desiccant is withinthe desired range. Once the time since initialization exceeds the presettime period (step 1507), the system controller (e.g., 605 or 607)activates the desiccant reactivation system. While the steps associatedwith the reactivation system may vary depending upon the exactconfiguration of the dehumidifier system as noted above, in at least oneembodiment during desiccant reactivation the airflow between thedehumidifier system and the battery pack is closed (step 1509), theheating system is activated (step 1511), and ambient air is forcedthrough the desiccant (step 1513). After a preset period of time hasexpired (step 1515), the flow of ambient air is terminated (step 1517),the heater is turned off (step 1519), and the airflow between thedehumidifier system and the battery pack is opened (step 1521). At thispoint the system is re-initialized (step 1523) and the system re-entersnormal operation.

FIG. 16 illustrates a minor modification of the process shown in FIG.15. As shown, after the heating system is activated (step 1511), thesystem waits for a preset time period (step 1601) to allow heaterwarm-up prior to opening the desiccant container to ambient airflow.

In the process illustrated in FIG. 17, once the system determines thatthe time since the last reactivation has exceeded the preset time period(i.e., t₁ in FIG. 17) the system starts monitoring for the next timeperiod in which the vehicle is out-of-service (step 1701). The systemcan be set-up to associate the vehicle being out-of-service with variouscriteria. For example, in one configuration an out-of-service vehiclecorresponds to the vehicle being parked and/or in the ‘off’ state (e.g.,driver not in the driver's seat; key/key fob not in proximity to thevehicle; etc.). In another configuration an out-of service vehiclecorresponds to a preset time of day (e.g., 3 AM). In yet anotherconfiguration an out-of-service vehicle corresponds to the battery packbeing coupled to a charging system and an external power source. Oncethe system controller determines that the vehicle is out-of-service(step 1703), the desiccant reactivation process is initiated asdescribed relative to either FIG. 15 or FIG. 16.

It will be appreciated that initiation of the desiccant reactivationsystem of the present invention may be based on other criteria than thetime since the last reactivation. For example, the humiditycorresponding to a region of the battery pack and/or a region of thedesiccant container may be monitored and used to determine when thedesiccant has become saturated and desiccant reactivation is required.This approach is illustrated in FIGS. 18-20 which correspond to FIGS.15-17 with the exception that instead of monitoring time, theseembodiments monitor humidity (step 1801) and only activate the desiccantreactivation system when the detected humidity exceeds a preset level(step 1803).

It should be understood that identical element symbols used on multiplefigures refer to the same component, or components of equalfunctionality. Additionally, the accompanying figures are only meant toillustrate, not limit, the scope of the invention and should not beconsidered to be to scale.

Systems and methods have been described in general terms as an aid tounderstanding details of the invention. In some instances, well-knownstructures, materials, and/or operations have not been specificallyshown or described in detail to avoid obscuring aspects of theinvention. In other instances, specific details have been given in orderto provide a thorough understanding of the invention. One skilled in therelevant art will recognize that the invention may be embodied in otherspecific forms, for example to adapt to a particular system or apparatusor situation or material or component, without departing from the spiritor essential characteristics thereof. Therefore the disclosures anddescriptions herein are intended to be illustrative, but not limiting,of the scope of the invention which is set forth in the followingclaims.

What is claimed is:
 1. A battery pack dehumidifier system, comprising: abattery pack, comprising a battery pack enclosure configured to hold aplurality of batteries; and a dehumidifier assembly, comprising: adesiccant enclosure; a desiccant contained within said desiccantenclosure; a first air passageway coupling said desiccant enclosure tosaid battery pack enclosure, wherein said first air passageway allowswater vapor to pass from said battery pack enclosure and into saiddesiccant enclosure where said water vapor is absorbed and adsorbed bysaid desiccant; a controller configured to estimate when said desiccantloses absorption/adsorption efficacy, wherein said processor is furtherconfigured to output a first control signal when said desiccant losesabsorption/adsorption efficacy; a heater assembly, wherein said heaterassembly is configured to heat and reactivate said desiccant within saiddesiccant enclosure upon receipt of said first control signal, whereinduring heating and reactivation of said desiccant moisture is releasedfrom said desiccant; and a second air passageway coupling said desiccantenclosure to a volume of air outside of said battery pack enclosure,wherein said moisture released during desiccant heating and reactivationpasses through said second air passageway; wherein said heater assemblyfurther comprises a heating element positioned within a heating chamberof said desiccant enclosure, wherein said heating chamber is separatefrom a desiccant chamber within said desiccant enclosure, wherein saiddesiccant is contained within said desiccant chamber; and wherein saidheater assembly further comprises a plurality of thermally conductivestructures that are positioned within said desiccant and throughout saiddesiccant chamber, wherein said plurality of thermally conductivestructures are heated by said heating element during heating andreactivation of said desiccant, and wherein said plurality of thermallyconductive structures transfer heat from said heating element to saiddesiccant.
 2. The battery pack dehumidifier system of claim 1, furthercomprising a plurality of valves, wherein said plurality of valves closesaid first air passageway and open said second air passageway duringheating and reactivation of said desiccant, and wherein said pluralityof valves open said first air passageway and close said second airpassageway during pre- and post-heating and reactivation of saiddesiccant.
 3. The battery pack dehumidifier system of claim 1, whereinsaid second air passageway couples said desiccant enclosure to anambient environment, wherein said volume of air outside of said batterypack enclosure consists of said ambient environment.
 4. The battery packdehumidifier system of claim 1, wherein said controller is configured tooutput a second control signal after a preset period of time has passedsince outputting said first control signal, and wherein said heaterassembly is configured to terminate heating and reactivation of saiddesiccant within said desiccant enclosure upon receipt of said secondcontrol signal.
 5. The battery pack dehumidifier system of claim 1,further comprising at least one humidity sensor coupled to saidcontroller, wherein said controller outputs said first control signalwhen a humidity level detected by said at least one humidity sensorexceeds a preset humidity level.
 6. The battery pack dehumidifier systemof claim 5, wherein said at least one humidity sensor is mounted withinsaid battery pack enclosure.
 7. The battery pack dehumidifier system ofclaim 5, wherein said at least one humidity sensor is mounted withinsaid desiccant enclosure.
 8. The battery pack dehumidifier system ofclaim 1, further comprising a timer, wherein said controller outputssaid first control signal after a preset period of time has passed sinceinitialization of said dehumidifier system.
 9. The battery packdehumidifier system of claim 1, further comprising a timer, wherein saidcontroller outputs said first control signal after a preset period oftime has passed since said controller last output said first controlsignal.
 10. The battery pack dehumidifier system of claim 1, whereinsaid heater assembly further comprises a heating element positionedwithin said desiccant contained within said desiccant enclosure.
 11. Thebattery pack dehumidifier system of claim 10, wherein said heaterassembly further comprises a fan, wherein said fan circulates airthroughout said desiccant during heating and reactivation of saiddesiccant.
 12. The battery pack dehumidifier system of claim 10, whereinsaid heater assembly further comprises a plurality of thermallyconductive structures that are positioned within said desiccant andthroughout said desiccant chamber, wherein said plurality of thermallyconductive structures are heated by said heating element during heatingand reactivation of said desiccant, and wherein said plurality ofthermally conductive structures transfer heat from said heating elementto said desiccant.
 13. The battery pack dehumidifier system of claim 1,wherein said heater assembly further comprises a heating element,wherein said heating element is positioned within a heating chamberseparate from said desiccant enclosure, wherein air heated within saidheating chamber is driven through said desiccant enclosure duringheating and reactivation of said desiccant.
 14. The battery packdehumidifier system of claim 13, wherein said heater assembly furthercomprises a fan, wherein said fan drives air heated within said heatingchamber through said desiccant and said desiccant enclosure duringheating and reactivation of said desiccant.
 15. The battery packdehumidifier system of claim 1, wherein said heater assembly furthercomprises a fan, wherein said fan drives air heated within said heatingchamber of said desiccant enclosure through said desiccant and saiddesiccant chamber during heating and reactivation of said desiccant. 16.The battery pack dehumidifier system of claim 1, wherein each of saidplurality of thermally conductive structures consists of a fin extendingfrom the heating chamber into the desiccant.
 17. The battery packdehumidifier system of claim 1, wherein said plurality of thermallyconductive structures are aligned with said first air passageway tominimize disruption of airflow between said battery pack enclosure andsaid desiccant enclosure.
 18. The battery pack dehumidifier system ofclaim 1, further comprising at least one fan that circulates air betweena first internal volume corresponding to said battery pack enclosure anda second internal volume corresponding to said desiccant enclosure. 19.The battery pack dehumidifier system of claim 1, wherein said desiccantenclosure is contained within said battery pack enclosure.
 20. Thebattery pack dehumidifier system of claim 1, wherein said desiccantenclosure is externally mounted to said battery pack enclosure, andwherein said first air passageway is defined by a pass-through betweensaid desiccant enclosure and said battery pack enclosure.
 21. Thebattery pack dehumidifier system of claim 1, wherein said desiccantenclosure is remote from said battery pack enclosure and coupled to saidbattery pack enclosure via said first air passageway.
 22. The batterypack dehumidifier system of claim 16, wherein the fins are arranged in arow with flat sides of the fins facing each other, and wherein the firstair passageway is configured so that air from the battery pack enclosureflows along the flat sides between adjacent fins.
 23. The battery packdehumidifier system of claim 1, wherein each of said plurality ofthermally conductive structures consists of a porous plate with aplurality of apertures.
 24. The battery pack dehumidifier system ofclaim 1, wherein each of said plurality of thermally conductivestructures has a crisscross pattern of thermally conductive elements.